1. Field of the Invention
The present invention relates to a method for transmitting Acknowledge/Negative-Acknowledge signal (ACK/NACK), which represents whether uplink data must be retransmitted, through a downlink common channel to use a Hybrid Automatic Repeat Request (HARQ) scheme according to an uplink enhancement in a Code Division Multiple Access (CDMA) mobile communication system using a Time Division Duplexing (TDD) scheme.
2. Description of the Related Art
3G mobile communication provides both voice and packet-data service using a CDMA scheme. The 3G mobile communication scheme may be classified into a Universal Mobile Telecommunication Service (UMTS) scheme and a CDMA 2000 scheme. The UMTS is a standard scheme in Europe and Japan, is based on synchronization between nodes Bs and conforms to a 3rd Generation Partnership Project (3GPP) standard. The CDMA 2000 is a standard scheme in the US, is based on asynchronization between nodesBs and conforms to a 3GPP2 standard.
In the 3G mobile communication, a 3GPP includes a Frequency Division Duplexing (FDD) scheme allowing uplink/downlink transmission/reception to be differentiated by frequency and a TDD scheme allowing uplink/downlink transmission/reception to be differentiated by time. Herein, the TDD scheme includes a High Chip Rate TDD (HCR-TDD) using a chip rate of 3.84 Mcps (Mega chip per second) and a Low Chip Rate TDD (LCR-TDD) using a chip rate of 1.28 Mcps.
In the 3G mobile communication scheme, with increasing demand for data-packet services via mobile communication and the increase in data size, a High Speed Downlink Packet Access (HSDPA) scheme has been proposed that transmits data more efficiently and rapidly using the UMTS scheme. In addition, standardization for the HSDPA scheme has been discussed in the 3GPP. Under the HSDPA scheme, it is possible to efficiently transmit data through a downlink at a high speed. However, there is also a need for a faster data transmission rate through the uplink for interactive multimedia services.
In response, an uplink enhancement has been proposed to transmit data through the uplink at higher speeds. In addition, to improve the performance of packet transmission, a HARQ scheme is used together with the uplink enhancement. Furthermore, in order to use the HARQ scheme, control information must be transmitted downward. Herein, the control information is information sent from a node B to User Equipment (UE) and indicates whether data sent from the UE has been successfully received by the node B.
FIG. 1 is a diagram illustrating a HARQ scheme in an uplink enhancement.
Referring to FIG. 1, a UE 102 transmits data through an uplink enhancement channel 103 allocated by a node B 101. After the data is transmitted, the node B 101 determines whether the received data has an error. As a result of the determination, when the data have been received without an error, the node B 101 transmits an ACK to the UE 102. In contrast, when the data does have an error, the node B 101 transmits a NACK (104) to the UE 102. The ACK/NACK is transmitted through a downlink channel and the downlink channel may transmit only the ACK/NACK or transmit the ACK/NACK together with another signaling information. Then, when the ACK is received from the node B 101, the UE 102 transmits new data. In contrast, when the NACK is received from the node B 101, the UE 102 transmits the previously transmitted data (105).
To decrease the delay of data transmission when the HARQ scheme is used, a N-channel Stop and Wait (NSW) HARQ scheme of continuously transmitting N data blocks and waiting for the transmission responses in turn is used. The NSW HARQ scheme is controlled by a node B.
FIG. 2 is a diagram illustrating a conceptual operation of a NSW HARQ scheme when data are transmitted to a node B through an uplink.
Referring to FIG. 2, in a UE, data blocks transferred from an upper layer are stored in a buffer 201. The data stored in the buffer 201 are sequentially distributed to a number of HARQ processes 202 to a HARQ process 204. The number N of HARQ processes is determined considering a delay time it takes to receive a response when transmitting data between the UE and the node B. N may be equal to or greater than 2.
The data 206 processed in the HARQ process 202 is transmitted through an uplink enhancement channel during one Transmission Time Interval (TTI). After transmitting the data 206, the UE transmits the data 207 processed in the HARQ process 203 when the next TTI comes. In this way, the UE transmits the data processed in from the HARQ process 202 to the HARQ process 204, which is the last process, during the time interval N*TTI 211. Herein, during data transmission, the UE does not receive responses for the transmitted data. Further, while the data is continuously transmitted, the node B sequentially receives the data from the first transmitted data 206 to the last transmitted data 209.
When receiving the data 206, the node B judges whether the data 206 has an error. Herein, when the node B fails in a CRC (Cyclic Redundancy Check) examination of the data 206, the node B determines that the data 206 has an error. In contrast, when the node B succeeds in the CRC examination of the data 206, the node B determines that the data 206 is received without error. Then, the node B transmits an ACK/NACK 212 indicating whether an error has occurred in the data to the UE through a downlink channel 210 (212).
The ACK/NACK 212 is transmitted through the downlink channel 210 and must be transmitted within the N*TTI 211 after the first data 206 have been transmitted. The ACK/NACK determination is also performed for the next received data 207. In this way, the ACK/NACK determination is performed for each data sequentially transmitted from the N HARQ processes 202 to 204, so that the ACK/NACK is independently transmitted.
The ACK/NACK 213 for the data 207 is transmitted through the downlink channel 210 during the next TTI after the ACK/NACK 212 has been transmitted. When the UE receives the ACK/NACK 212 at a reception position of the downlink channel 210 corresponding to the data 206, the UE transmits new data or retransmission data in an interval 208 corresponding to the HARQ process 202 according to the ACK/NACK 212. The aforementioned operation is also performed for the ACK/NACK 213 and the ACK/NACK 213 takes part in whether the data 207 processed in the HARQ process 203 is retransmitted. That is, new/retransmission data according to the ACK/NACK 213 are transmitted in an interval 209 corresponding to the HARQ process 203.
The ACK/NACK transmitted through the downlink channel is information for determining whether an uplink data block should be retransmitted. If an error occurs in the ACK/NACK, correct data may be unnecessarily retransmitted or it may be impossible to transmit a data needing retransmission. Accordingly, the ACK/NACK is as important as the data block. Further, the ACK/NACK must have a very low Block Error Rate (BLER) as compared to the data block.
The ACK/NACK may be expressed by one bit representing an ACK or a NACK. The ACK or the NACK of one bit passes through a channel coding process in order to be transmitted at a low error rate. In the channel coding process, the ACK/NACK is carried by a plurality of coding bits, so that a receiver can receive the ACK/NACK at a low error rate. A repetition coding method may be used for a channel coding of the ACK/NACK. In the repetition coding, one bit is repeatedly transmitted by the number of bits to be transmitted. Repetition coding may be usefully utilized when information to be transmitted has only two types (e.g., 0 or 1), similarly to the transmission of the ACK/NACK, because a coding process in a transmitter and a decoding process in a receiver is simple and easy to implement.
The ACK/NACK having passed through the channel coding process is transmitted through a downlink physical channel to which a downlink channel is mapped. The downlink physical channel is allocated in advance when the node B and the UE start to communicate with each other and set to a radio link.
In a method for determining an uplink/downlink channel between the node B and the UE, from among time slots, a specific channelization code in time slots used for downlink transmission is set as a downlink channel and a specific channelization code in time slots used for uplink transmission is set as an uplink channel. The uplink/downlink channels having been set between the node B and the UE may be reset in a unit of each radio frame or set again after several radio frames. The repetition period of the uplink/downlink channels between the node B and the UE and the number of radio frames during an interval between resetting of the uplink/downlink channels may be determined according to the property and amount of data transmitted between the node B and the UE. For example, when the amount of uplink transmission data is large and the amount of downlink transmission data is small, the uplink channel may be frequently reset as compared to the downlink channel.
For uplink enhancement, many uplink channels are allocated and the downlink channels are minimally used. A downlink channel required for uplink enhancement is a channel for transmitting signaling information required for the uplink enhancement, and the required number of the downlink channels may change according to the property and amount of the signaling information. Accordingly, it is necessary to provide technology for effectively transmitting the ACK/NACK through a downlink channel as signaling information for the uplink enhancement.